1. Field of the Invention
The invention is related to the field of magnetoresistance (MR) read elements and, in particular, to MR read elements where one of the shields sandwiching an MR sensor forms an active portion of the MR sensor.
2. Statement of the Problem
Many computer systems use magnetic disk drives for mass storage of information. Magnetic disk drives typically include one or more recording heads (sometimes referred to as sliders) that include read elements and write elements. An actuator/suspension arm holds the recording head above a magnetic disk. When the magnetic disk rotates, an air flow generated by the rotation of the magnetic disk causes an air bearing surface (ABS) side of the recording head to fly a particular height above the magnetic disk. The height to which the recording head flies depends on the shape of the ABS. As the recording head rides on the air bearing, an actuator moves the actuator/suspension arm to position the read element and the write element over selected tracks of the magnetic disk.
To read data from the magnetic disk, transitions on a track of the magnetic disk emit magnetic fields. As the read element passes over the transitions, the magnetic fields of the transitions modulate the resistance of the read element. The change in resistance of the read element is detected by passing a sense current through the read element, and then measuring the change in bias voltage across the read element to generate a read signal. The resulting read signal is used to recover the data encoded on the track of the magnetic disk.
One type of read element uses magnetoresistance (MR) sensors to sense the transitions on the magnetic disk. The read element may use Giant MR (GMR) sensors, Tunneling MR (TMR) sensors, or other types of MR sensors. The basic structure of a read element includes an MR sensor (or MR sensor stack) sandwiched between two shields. The MR sensor is formed from a plurality of thin-films. The thin-films include an antiferromagnetic (AFM) pinning layer (e.g., PtMn), a ferromagnetic pinned layer (e.g., CoFe), a nonmagnetic spacer layer (e.g., Cu), and a ferromagnetic free layer (e.g., CoFe). The AFM pinning layer has a fixed magnetization that in turn fixes the magnetic moment of the pinned layer perpendicular (transverse) to the ABS of the read element. The pinned layer may be comprised of a single layer, or may have a synthetic antiferromagnetic (SAF) pinned structure. An SAF pinned structure includes a first ferromagnetic pinned (keeper) layer (e.g., CoFe), an antiparallel coupling layer (e.g., Ru), and a second ferromagnetic pinned (reference) layer (e.g., CoFe). The first pinned (keeper) layer has a magnetization oriented in a first direction perpendicular to the ABS by exchange coupling with the AFM pinning layer. The second pinned (reference) layer is antiparallel coupled with the first pinned (keeper) layer across the antiparallel coupling layer. Accordingly, the magnetization of the second pinned (reference) layer is oriented in a second direction that is antiparallel to the direction of the magnetization of the first pinned (keeper) layer.
When a read element is performing a read operation, the ABS of the MR sensor is positioned adjacent to a desired track on the magnetic disk. A transition on the magnetic disk will pass under the first shield, under the MR sensor, and then under the second shield. When the transition passes under the MR sensor, the magnetic fields of the transition will rotate the magnetic moment of the free layer, which in turn changes the resistance of the MR sensor. The change in resistance of the MR sensor is detected by the sense current that is passed through the MR sensor. The transitions sensed by the sense current thus provide the read signal representing the data written to the magnetic disk.
The purpose of the shields formed on either end of the MR sensor is to absorb magnetic fields emitted from neighboring transitions along the track during the read operation. The MR sensor thus only “sees” the magnetic fields from the transition which is presently underneath the MR sensor and not the magnetic fields from the neighboring transitions. If the read element is used in a current perpendicular to plane (CPP) fashion, then the shields may also be used as current leads for the sense current, as the current is injected from one shield, through the MR sensor (perpendicular to the major planes), and through the other shield. Two main types of CPP MR sensors can be distinguished, one is fully metallic and based on the GMR effect, the other contains an insulating barrier and is based on the tunneling magnetoresistance (TMR) effect.
As densities of the magnetic disk increase beyond 500 Gb/in2, the spacing between the shields needs to be lower than about 300 nanometers. To get spacing this low, the total thickness (i.e., the distance between the shields) of the MR sensor needs to be lower than 300 Å. It is presently a problem to fabricate a metallic CPP MR sensor having a thickness less than 300 Å while still providing an adequate read signal. Large MR sensor thicknesses provide optimal read signal performance, but at the same time the large MR sensor thicknesses do not allow for higher density recording. TMR based CPP sensors can more easily fit inside a 300 Å shield-to-shield spacing, but can present a similar problem at smaller spacing (i.e. higher linear densities).